With increasing depth of an oil well and a gas well (hereinafter, an oil well and a gas well are referred simply to as an “oil well” as a general term), the oil country tubular goods are required to have a higher strength. Conventionally, oil country tubular goods of 80 ksi grade (having a yield stress of 80 to 95 ksi, that is, 551 to 654 MPa) or 95 ksi grade (having a yield stress of 95 to 110 ksi, that is, 654 to 758 MPa) have been used widely. Nowadays, however, oil country tubular goods of 110 ksi grade (having a yield stress of 110 to 125 ksi, that is, 758 to 862 MPa) are put in use.
Further, many of the deep wells having been developed recently contain corrosive hydrogen sulfide. Therefore, the oil country tubular goods are required to have not only a high strength but also a sulfide stress cracking resistance (hereinafter, also referred to as an SSC resistance).
As a measure for improving the SSC resistance of the conventional oil country tubular goods of 95 to 110 ksi grades, there has been known a method in which a steel is purified, or a steel micro-structure is made fine. For example, JP62-253720A proposes a method for improving the SSC resistance by reducing impurity elements such as Mn and P. JP59-232220A proposes a method in which grains are made fine by performing quenching treatment twice to improve the SSC resistance.
As described above, to meet the requirement for higher strength of oil country tubular goods, nowadays, a steel for oil country tubular goods of 125 ksi grade (having a yield stress of 862 to 965 MPa) are proposed. However, with an increase in strength, the sulfide stress cracking (SSC) is liable to occur. Therefore, for the steel for oil country tubular goods of 125 ksi grade or higher, further improvement is required for the SSC resistance as compared with the conventional steel for oil country tubular goods of 95 ksi or 110 ksi grade.
JP6-322478A, JP8-311551A, JP11-335731A, JP2000-178682A, JP2000-256783A, JP2000-297344A, JP2000-119798A, JP2005-350754A, and JP2006-265657A propose measures for improving the SSC resistance of high-strength steels for oil country tubular goods.
JP6-322478A proposes a method in which a steel micro-structure is made fine by induction heating heat treatment to improve the SSC resistance of a steel material of 125 ksi grade. JP11-335731A proposes a method in which the hardenability is enhanced when using the direct quenching process and the tempering temperature is increased to improve the SSC resistance of steel pipes of 110 ksi grade to 140 ksi grade. JP11-335731A proposes a method for improving the SSC resistance of a low alloy steel of 110 ksi grade to 140 ksi grade by regulating the alloy elements so as to have optimal contents. JP2000-178682A, JP2000-256783A and JP2000-297344A propose methods for improving the SSC resistances of low alloy steels for oil country tubular goods of 110 ksi grade to 140 ksi grade by controlling the mode of carbide. JP2000-119798A proposes a method for delaying the time of occurrence of SSC in a steel material of 110 ksi grade to 125 ksi grade by precipitating fine V carbides in large amounts. JP2005-350754A proposes a method for improving the SSC resistance of oil country tubular goods of 125 ksi grade or higher by controlling the dislocation density and the hydrogen diffusion coefficient to desired values. JP2006-265657A proposes a method in which a bainite single phase structure containing much carbon is formed by stopping water cooling at 400 to 600° C. at the water cooling time and by performing the isothermal transformation heat treatment (austemper treatment) at 400 to 600° C. to improve the SSC resistance of a steel for oil country tubular goods of 125 ksi grade or higher.